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CHEM 101-GENERAL CHEMISTRY CHAPTER 7

CHEMICAL BONDING & MOLECULAR STRUCTURE

INSTR : FİLİZ ALSHANABLEH

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CHAPTER 7 CHEMICAL BONDING & MOLECULAR STRUCTURE

• The Ionic Bond

• Formation of Ions

• The Covalent Bond

• Electronegativity and Bond Polarity

• Lewis Structures

• Shapes of Molecules

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The Ionic Bond

• An ionic bond is formed by the electrostatic attraction of oppositely charged ions. • Ionic compounds form between metals and nonmetals.

• The greater the difference in metallic/nonmetallic

properties (more widely separated in the periodic table), the more likely it is a compound will be ionic.

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Formation of Cations

• Metals in the s and p blocks have low ionization energies and form cations with an np6 electronic configuration.

20Ca ([Ar] 4s2) Ca2+( [Ar] ) + 2e-

• Cations are smaller than their corresponding neutral atoms. • Losing electrons reduces electron-electron repulsion. • Remaining electrons are more tightly bound to the nucleus.

• Transition metals can form cations with more than one possible

charge. • Transition metals first lose electrons from the s subshell. • Additional electrons are lost from the partially filled d orbitals. • Fe2+ and Fe3+ ions are both stable. 26Fe ([Ar] 4s2 3d6) Fe2+([Ar] 3d6) + 2e- Fe3+( [Ar] 3d5 ) + e-

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Formation of Anions

• Nonmetals have negative electron affinities and generally form anions with an np6 electronic configuration.

16S ( [Ne] 3s2 3p4) + 2e- S2- ([Ar)] • Anions are larger than their corresponding neutral atoms.

• Gaining electrons increases electron-electron repulsion.

• Valence electrons less tightly bound to the nucleus.

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Formation of Ions

• Sizes of ions compared to corresponding neutral atoms.

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Formation of Ions

• Cations decrease in size from left to right across the periodic table. • Cations in a period have the same electronic

configuration.

• Anions decrease in size from left to right across the periodic table. • Anions in a period have the same electronic configuration.

• Number of protons increases from left to right across a period.

• Electrons are held more tightly from left to right across a

period, resulting in smaller ions.

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Formation of Ions

• Forming an ionic bond between a metal and nonmetal usually requires energy to form the ion pair.

• Ionization energies are positive.

• Electron affinities for nonmetals are negative.

• Energy input to form the cation is not offset by energy

released by forming the anion.

• Once an ion pair is formed, electrostatic force of attraction between the ions significantly lowers overall energy.

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Formation of Ions

• The energy released in forming NaF can be calculated. • First the ion pair must be formed. (496 – 328 = +168 kJ/mol)

• The ionization energy for Na is 496 kJ/mol. • The electron affinity for F is -328 kJ/mol.

• Second the potential energy from coulombic attraction (between

Na+ and F- ions) is calculated. Use the ionic radii to calculate V. • V = -591 kJ/mol.

• The energy released from the coulombic attraction is much greater

than the energy required to form the NaF ion pair. • The formation of the NaF ionic bond releases energy.

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Formation of Ions

• In a solid lattice, any given ion experiences a large number of attractive and repulsive interactions.

• In ionic solids, the ions are arranged in a crystal lattice. • Ions experience attractive and repulsive interactions in three

dimensions.

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Formation of Ions • The lattice energy is the overall result of the attractive and repulsive forces

a crystal contains. • Once an ion pair is formed, electrostatic force of attraction between the

ions significantly lowers overall energy.

• F is force of attraction, q1 and q2 are the charges, and r is the distance between the nuclei of the two ions.

• The potential (lattice) energy, V, for the ion pair can be calculated.

• Small ions with large charges form ionic compounds with large lattice energies.

• Large ions with small charges form ionic compounds with small lattice energies.

F ∝q1q2

r2

V = k q1q2

r

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Example Problem 7.2

• In each of the following pairs of ionic solids, the ions are arranged into similar lattices. Which one has the largest lattice energy? • CaO or KF

• NaF or CsI

• NaF or MgF2 O F Na Mg K Ca Cs

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The Covalent Bond

• A covalent bond is based on the sharing of pairs of electrons between two atoms.

• Driving force behind bond formation is lowering of overall energy. • Ionic bonding lowers energy by transferring electrons

between a metal and a nonmetal.

• Covalent bonding lowers energy by sharing electrons between two nonmetals.

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Chemical Bonds and Energy

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Chemical Bonds and Energy

• Bond energy - energy released when isolated atoms form a covalent bond.

• Bond length - the distance between the nuclei of the bonding atoms where the potential energy is a minimum.

When two atoms approach one another, the negatively charged electron clouds are attracted to the other atom’s positively charged nucleus.

The diagram represents electron density during bond formation.

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Chemical Bonds and Energy

• Formation of bonds always releases energy. • Once a bond is formed, the same amount of energy, the bond

energy, is needed to break the bond apart. • Bond energies vary depending on the bonding atoms involved. Chemical reactions involve rearranging bonds, turning reactants into products.

Reactions are energetically favored if the energy required to break reactant bonds is less than energy released making product bonds.

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Chemical Bonds and the Structure of Molecules

• During ionic bond formation, the cations and anions achieve np6 electronic configurations (noble gas configuration). • Metals lose electrons. • Nonmetals gain electrons.

• During covalent bond formation, electrons are shared between two atoms. • Shared electrons are available to both bonding atoms. • Sharing leads to 8 valence electrons around each atom.

• Octet rule - an atom will form covalent bonds to achieve a

complement of eight valence electrons. • The valence shell electronic configuration is ns2np6 for a total of

eight electrons. • For the n = 1 shell, hydrogen violates the octet rule and shares

only 2 electrons.

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Chemical Bonds and the Structure of Molecules

• Lewis dot symbols keep track of valence electrons, especially for main group elements, allowing prediction of bonding in molecules. • To draw a Lewis dot symbol, the valence electrons are

represented by dots and are placed around the element symbol.

• The first four dots are placed singly. • Starting with the fifth dot, they are paired.

• The second period Lewis symbols are shown below:

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Chemical Bonds and the Structure of Molecules

• Lewis dot symbols for main group elements. • Elements within a group have the same number of

valence electrons and identical Lewis dot symbols.

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Chemical Bonds and the Structure of Molecules

• Lewis dot structures show how electrons are shared in a molecule. • A pair of shared electrons between two atoms is a bonding

pair.

• Bonding pairs represented by a line between two atomic symbols.

• Pairs of electrons associated with one atom are nonbonding or lone pair electrons.

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Chemical Bonds and the Structure of Molecules

• By sharing an electron from each atom, two hydrogen atoms can form a covalent bond. • Hydrogen violates the octet rule by sharing only two

electrons.

• When two fluorine atoms combine, they form a stable covalent bond. • By sharing a pair of electrons, each atom is surrounded

by eight valence electrons.

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Chemical Bonds and the Structure of Molecules

• Bonding atoms in molecules can share more than one bonding pair of electrons. • A double bond results when two bonding pairs are shared.

• A triple bond results when three bonding pairs are shared.

• Strength of the covalent bond increases as the number of

bonding pairs increases.

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Electronegativity and Bond Polarity

• Bonding between the two ends of the bonding continuum, ionic and covalent bonding, is described using electronegativity and bond polarity.

• Electronegativity is the attraction of an atom for the shared

electrons in a covalent bond. • Electronegativities are not measured quantities. • Electronegativities are assigned based on factors such as

atomic size, electron affinity, and ionization energy. • The higher the electronegativity value, the more likely an

element will attract extra electron density during compound formation.

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Electronegativity

• Electronegativities increase from left to right across a period and from bottom to top for a group.

• Fluorine is the most electronegative element, with an electronegativity of 4.0.

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Bond Polarity

• The more electronegative element :partial negative charge. • The less electronegative element: partial positive charge. • The two points of (+)ve & (-)ve charge constitute a dipole.

• A bond along which a dipole exists is a polar bond.

• The greater the electronegativity difference, the more polar the

bond.

• When the electronegativity difference is zero, the bond is classified as nonpolar covalent.

• When the electronegativity difference exceeds 2.0, the bond is classified as ionic.

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Bond Polarity

• The formation of the polar covalent HF bond. • The more electronegative F has a partial negative charge. • The less electronegative H has a partial positive charge.

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Problem 7.35

• In each group of three bonds, which bond is likely to be the most polar?

(a) C–H, O–H, H–S (b) C–Cl, Cl–Cl, H–Cl (c) F–F, O–F, C–F (d) N–H, N–O, N–Cl (Electronegativities: H:2.1, C:2.5, O:3.5, S:2.5, Cl:3.0, F:4.0, N:3.0) Difference in electronegativities (a) C–H(0.4), O–H(1.4 - the most polar), H–S(0.4) (b) C–Cl(0.5), Cl–Cl(0.0- nonpolar), H–Cl (0.9 – the most polar) (c) F–F(0.0- nonpolar), O–F(0.5), C–F(1.5 – the most polar) (d) N–H(0.9 – the most polar), N–O(0.4), N–Cl (0.0- nonpolar)

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Keeping Track of Bonding: Lewis Structures

• Lewis structures indicate how many bonds are formed and between which elements in a compound.

• Step 1 - Count the total valence electrons in the molecule or ion. • Sum the number of valence electrons for each element in a

molecule. • For ions, add or subtract valence electrons to account for the

charge. Example: Draw Lewis structure for the compound OF2,

8O : 1S22S22P4 (6 valence e-)

9F : 1S22S22P5 (2x7=14 valence e-)

the number of valence electrons OF2 is 20.

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Keeping Track of Bonding: Lewis Structures

• Step 2 - Draw the “skeletal structure” of the molecule. • The element written first in the formula is usually the central atom, unless it is hydrogen. • Usually, the central atom is the least electronegative.

• Step 3 - Place single bonds between all connected atoms in the structure by drawing lines between them. • A single line represents a bonding pair. • Four electrons are placed in bonds. • Sixteen electrons are left to place.

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Keeping Track of Bonding: Lewis Structures

• Step 4 - Place the remaining valence electrons not accounted for on individual atoms until the octet rule is satisfied. Place electrons as lone pairs whenever possible. • Place electrons first on outer atoms, then on central

atoms. • Six electrons are placed as lone pairs on each F

satisfies the octet rule for each F. • The four remaining electrons are placed on the O to

satisfy the octet rule for each O.

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Keeping Track of Bonding: Lewis Structures

• Step 5 - Create multiple bonds by shifting lone pairs into bonding positions as needed for any atoms that do not have a full octet of valence electrons. • Correctly choosing which atoms to form multiple bonds

between comes from experience. • Multiple bonds are not required for OF2, as the octet rule is

satisfied for each atom.

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Exercise

• Draw a Lewis structure for each of the following molecules: H2O, PBr3, and HCN

• H2O: 2x1+6 = 8e-

• PBr3: 5+(3x7)= 26e-

• HCN: 1+6+5= 12e-

O HH

P

BrBr Br

H C N

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Problem 7.42

• Draw a Lewis structure for each of the following molecules or ions: (a) CS2, (b) BF4

–, (c) HNO2 (where the bonding is in the order HONO), (d) OSCl2 (where S is the central atom)

• (a) CS2, 4+ (2x6)= 16e-

• (b) BF4–, 3+ (4x7)+1= 32e-

• (c) HNO2 , 1+ 5+ (2x6) = 18e-

• (d) OSCl2 ,6+6+(2x7) =26e-

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Resonance

• Resonance structures can be drawn when the choice of multiple bond location is arbitrary. • The position of all atoms are identical; only the positions of the

electrons are different. EXAMPLE: SO2 has a total of 18 valence electrons (6+2x6). • S is the least electronegative element and the central atom.

• Octet not satisfied for S. • A double bond can be made to satisfy the octet rule, but from

which side?

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Resonance

• The two resonance structures for SO2. • The “real” structure is a hybrid of the two structures.

• The double arrow indicates the structures are resonance

structures.

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Exercise

• More than one valid Lewis structure can be written for a particular molecule.

NO3– : (5+ (3x6)+1) = 24e–

NO

O

ON

O

O

ON

O

O

O

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Resonance

• The two resonance structures for benzene. • Indicates the two ways to distribute 3 double bonds

among 6 carbon atoms. • The true structure is an average of the two resonance

structures.

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Hybrid Orbitals

• Hybrid orbitals are created by a linear combination of atomic orbitals, producing an equal number of hybrid orbitals. • Orbitals are mathematical in nature.

• Two atomic orbitals combine, two hybrid orbitals are

generated.

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Hybrid Orbitals

• Hybridization of the s and p orbitals on carbon. • The four sp3 hybrid orbitals have equal energy. • The four valence electrons are distributed evenly

across the sp3 hybrid orbitals. • The angle between the sp3 hybrid orbitals is 109.5o. Hybrid orbital name comes from the type and number of atomic orbitals combined (e.g., sp3)

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Shapes of Molecules

• Molecular shape - the way bonded atoms arrange themselves in three dimensions. • Molecular shape affects molecular properties, including

reactivity.

• Valence Shell Electron Pair Repulsion (VSEPR) theory - molecules assume a shape that allows them to minimize the repulsions between electron pairs in the valence shell of the central atom. • Electron pairs include both lone pair electrons and

bonding pair electrons.

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Shapes of Molecules

Steps to Apply the VSEPR Model 1. Draw the Lewis structure for the molecule. 2. Count the electron pairs and arrange them in the way that

minimizes repulsion (put the pairs as far apart as possible. 3. Determine the positions of the atoms from the way electron

pairs are shared (how electrons are shared between the central atom and surrounding atoms).

4. Determine the name of the molecular structure from positions of the atoms.

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Shapes of Molecules

• Bond angles and shapes are predicted by electron pair repulsion between valence electron pairs.

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Shapes of Molecules

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Example Problem 7.6

• Determine the shape of each of the following species: • PO4

3– (5+ 4x6 + 3 = 32e-) Tetrahedral

• PCl5 (7+ 5x7 = 42e-) Trigonal bipyramidal

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Shapes of Molecules

• Molecular shapes for various combinations of bonding and nonbonding electron pairs.

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Shapes of Molecules

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Problem 7.72

• Predict the geometry of the following species: (a) SO2, (b) BeCl2, (c) SeCl4, (d) PCl5 (a) SO2, (6+ 2x6 = 18e-) [ 3 1 ] Bent (120o) (b) BeCl2 (2+ 2x7 = 16e-) [ 2 0 ] Linear (180o) (c) SeCl4 (6+ 4x7 = 34e-) [ 4 1 ] Trigonal pyramidal (d) PCl5 (5+ 5x7 = 40e-) [ 5 0 ] Trigonal bipyramidal